In the manufacture of Portland cement, a factor which has assumed greater importance in recent years is the control of alkali content in the cement clinker. As is conventional in the art, what is meant by alkali herein is either Na.sub.2 O or K.sub.2 O. The presence of alkali in the cement clinker can result in what is known as the alkali-aggregate reaction, an effect which causes excessive expansive action of the concrete and serious weakening of the structure involved.
Various techniques have been used to obtain low total alkali content of the clinker which may be classed as follows:
1. Raw material control. PA1 2. Flue dust recycle control. PA1 3. Process parameter modification PA1 4. Raw mix additives.
A. The use of raw materials which are themselves of low alkali content. PA2 B. Leaching soluble alkali salts from the raw materials themselves. PA2 A. Discarding the flue dust. PA2 B. Leaching out soluble alkali salts from the flue dust prior to recycling it to the raw mix. PA2 A. Increasing the temperature of the burning process. PA2 B. Increasing the time period of the burning process. PA2 C. Modifying the cooling process.
An effective agent which has been found is CaCl.sub.2.2H.sub.2 O although to be effective its use can be costly. Probably the most effective agents currently in use are those containing hydrochloric acid. i.e. certain pickling liquors. When calcium chloride dihydrate is used it is added to the raw feed mix which, as is conventional, may be in dry or in slurry form. Likewise, hydrochloric acid when used is also added to the raw feed.
Conventionally, the mix progresses to the hotter regions of the kiln where the organic matter burns away and the carbonates lose carbon dioxide, a portion of the sulfates also is decomposed, liberating SO.sub.3, and chloride and alkali salts are partially volitalized. It is principally in this region of the kiln, before a portion of the mass is converted to liquid, where alkali loss is effected. It should be noted, in this respect, that even without the addition to the raw feed mix of material calculated specifically to reduce alkali content, one may expect on the average a loss of about 53% of the K.sub.2 O and 19% of the Na.sub.2 O incidental to the burning process, reaching final values typically less than 1% of the clinker.
Significant reduction in alkali content by the use of raw feed mix additives requires an economically significant amount of the additive even though the percentage amount of additive in the raw feed mix is low. This latter fact creates an additional problem in that the additive, to be effective must be very uniformly dispersed throughout the mix and adequate control is difficult to achieve.
Nevertheless, the use of additives to reduce alkali content of the clinker is widely practiced if for no other reason than to meet the ASTM standards for cement.